lib, zebra: Ensure route encoding has enough space

lib, zebra: Ensure route encoding has enough space

When you have compiled FRR with a large multipath number
then encoding large ecmp routes between zebra and the
routing daemons.  There exists a theoritical size
of multipath that will cause the encoding to be larger
than the ZEBRA_MAX_PACKET_SIZ.  In the cases where
we have allocated streams that will encode routes
then let's ensure that whatever size we have will
auto-fit what we say we can send.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>